Repression of photosynthetic genes by increased soluble carbohydrate concentrations may explain acclimation of photosynthesis to elevated CO, concentration. This hypothesis was examined in a field crop of spring wheat (Triticum aestivum L.) grown at both ambient (approximately 360 pmol mol-') and elevated (550 pmol mol-') atmospheric CO, concentrations using free-air CO, enrichment at Maricopa, Arizona. The correspondence of steady-state levels of mRNA transcripts (coding for the 83-kD photosystem I apoprotein, sedoheptulose-1,7-bisphosphatase, phosphoribulokinase, phosphoglycerokinase, and the large and small subunits of ribulose-l,5-bisphosphate carboxylase/oxygenase) with leaf carbohydrate concentrations (glucose-6-phosphate, glucose, fructose, sucrose, fructans, and starch) was examined at different stages of crop and leaf development and through the diurna1 cycle. Overall only a weak correspondence between increased soluble carbohydrate concentrations and decreased levels for nuclear gene transcripts was found. The difference in soluble carbohydrate concentration between leaves grown at elevated and current ambient CO, concentrations diminished with crop development, whereas the difference i n transcript levels increased. In the flag leaf, soluble carbohydrate concentrations declined markedly with the onset of grain filling; yet transcript levels also declined. The results suggest that, whereas the hypothesis may hold well i n model laboratory systems, many other factors modified its significance in this field wheat crop.Short-term exposure of terrestrial plants to [CO,] above the current ambient concentration of about 360 pmol mol-' causes an immediate increase in photosynthesis and primary production in C, species (Kramer, 1981;Cure and Acock, 1986). However, growth for prolonged periods at elevated [CO,] may result in physiological acclimation involving a decrease in their capacity for CO, assimilation (Peet et al., 1986;Sage et al., 1989;Yelle et al., 1989aYelle et al., , 1989b that is frequently associated with increased starch and soluble carbohydrate concentration in the source leaves Sasek et al., 1985;Stitt, 1991;Long and Drake, 1992). Severa1 lines of indirect evidence suggest that regulation of the expression of photosynthetic genes, via increased soluble carbohydrate concentration, may underlie acclimation to growth in elevated [CO,] (Sheen, 1990, 1994Stitt, 1991;Krapp et al., 1993;Webber et al., 1994). Decreased expression of severa1 photosynthetic genes has occurred when sugar concentrations have been increased manipulatively by directly feeding mature spinach leaves through the transpiration stream (Krapp et al., 1991), by expression of yeast-derived invertase in transgenic tobacco plants, which directs the gene product to the cell wall to interrupt export from source leaves (Stitt, 1991), and by cooling the petiole to decrease the rate of phloem transport in intact tobacco plants ( Krapp et al., 1993). Sheen (1990) demonstrated a possible mechanism for this by showing that addition of solub...
